Electric igniter system

ABSTRACT

An electric igniter system for a fuel burner including an electrical resistance type igniter, wherein the resistance element is of a type having a negative temperature coefficient and is coupled to a thermistor having a positive temperature coefficient. An electrically operated fuel valve is energized only after the igniter and thermistor resistances have been changed by heat from the igniter to values corresponding to the generation of fuel ignition temperatures by the igniter.

United States Patent [72] Inventor James R. Willson Garden Grove, Calif.[2]] Appl. No. 845,954 [22] Filed July 30, 1969 [45] Patented Sept. 28,1971 73] Assignee Robertshaw Controls Company Richmond, Va.

[54] ELECTRIC lGNlTER SYSTEM 13 Clalms, 4 Drawing Figs.

|52| US. Cl 431/66, 317/98, 431/254 [5 I Int. Cl F23n 5/14 [50] Field ofSearch 431/66, 24, 6, 254; 317/98, 79; 123/32 [56] References CitedUNITED STATES PATENTS 2,316,910 4/1943 Weber 431/66 X 2,549,633 4/1951Ottmar 431/66 X 2,621,647 12/1952 Pace 431/66 X 3,151,661 10/1964Matthews.. 431/66 X 3,282,324 11/1966 Romanelli. 431/24 3,454,345 7/1969Dyre 431/66 Primary Examiner-Frederick L. Matteson AssistantExaminerRobert A. Dua

Attorneys-Auzville Jackson, Jr., Robert L, Marben and Anthony A. OBrienIBURNER! :JLFUEL VALVE -FUEL INPUT 26 ATENIfinsePzsmam :2 609 ,072

I 26 BURNER 5x56 -FUEL INPUT INVENTOR JAMES R. WILLSON ATTORNEY ELECTRICrcurrsa SYSTEM BACKGROUND OF THE INVENTION l. Field of the Invention Thepresent invention relates to electric igniter systems for fuel burnersand, more particularly, to an igniter system wherein an electric igniterelement having a negative temperature coefficient of resistance isutilized in conjunction with a thermistor having a positive temperaturecoefficient to provide fail-safe operation.

2. Description of the Prior Art Devices such as the bimetallic elementhave been employed quite satisfactorily as heat sensors in systemsemploying pilot flame igniters to prevent raw fuel leakage into the areasurrounding the system burner upon pilot flame outage. However, certaindrawbacks soon become apparent; for example, pilot flame outage due todust particle buildup was found to be common, and the slow switchingtime of the bimetallic sensors often caused raw fuel leakage therebycreating a potentially dangerous condition.

Electric igniter systems have proven to be both reliable and efficientand have the ability to operate under the control of rapid actingelectrical safety circuits, thereby eliminating a substantial hazard. Insome instances, practical electric igniter elements having negativetemperature coefficients of resistance have been utilized for ignitionand control of fuel flow. While such igniter systems have particularadvantages in their respective arrangements, it is desirable to developigniter systems having increased safety, simplicity of operation andeconomy of manufacture.

SUMMARY OF THE INVENTION It is, therefore, an object of the presentinvention to construct an electric ignition system having all theadvantages of similarly employed prior art systems but utilizing fewercomponents than the components essential to such systems.

Another object of this invention is to provide an electric ignitersystem employing an igniter element having a negative temperaturecoefficient of resistance as both an igniting means and a heat sensingmeans.

This invention has a further object in the provision of an electricignition system having fail-safe characteristics assuring fuel ignition.

An additional object of the present invention is the provision of anelectric ignition system having raw fuel leakage preventioncharacteristics in the event of either an open circuit or a shortcircuit in the igniter element.

An advantage of the invention is the provision of a simple and reliableelectric igniter system having few parts and inherent safetycharacteristics.

The present invention is summarized in that an electric ignition systemfor a fuel burner includes an electrical power source coupled to anelectric igniter having a negative temperature coefficient of resistancewhich is adapted to be located within igniting proximity of the burner.The system further includes a resistor adapted to be heated by theigniter and having a positive temperature coefiicient, and anelectrically operated valve adapted to control a flow of fuel to theburner. The igniter, resistor and valveare coupled to a circuit which isresponsive to the resistance of the igniter and the resistor forenabling energization of the valve,v to thereby establish a flow offuel, only when the igniter is above fuel igniting temperatures and fordisabling the valve when the igniter develops either an electrical shortcircuit or an electrical open circuit.

These and other objects and advantages of the present invention willbecome more fully apparent from the following detailed description ofthe preferred embodiments of the invention when considered inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic circuit diagramof an electric igniter system embodying the present invention;

FIG. 4 is a schematic circuit the circuit illustrated in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS diagram of a modification ofFIG. 1, which illustrates a preferred embodiment of the presentinvention, includes an electric igniter 10 having a negative temperaturecoefficient of resistance, connected in series with a positivetemperature coefficient resistor 12 and a switch 14. The entire seriescircuit is connected to a source of electrical power, illustrated forsimplicity by lines 16. The energization winding 18 of anelectromagnetic relay 20 is connected in parallel with resistor 12 whilethe normally open relay contacts 22, which are mechanically biased to anopen position (as illustrated) in the absence of electrical energizationof the winding 18, are coupled in series with an electrically operatedfuel valve 24. The series circuit formed by switch 22 and valve 24 iscoupled in parallel with igniter 10 to complete the electrical circuit.

Fuel valve 24 is mechanically coupled in the fuel line 26 so as tocontrol the flow of fuel from the fuel source to the burner 28 that issuitably supported in conventional burner apparatus 30. The igniter 10is located within igniting proximity of the burner 28 while resistor 12is adapted to be positioned ad- 30 jacent the igniter so that thetemperatures of the igniter and the resistor are maintainedapproximately equal. It is noted that while the physical location ofigniter 10, resistor 12 and burner 28, with respect to each other, isthe same for all embodiments of the present invention, it has beenschematically illustrated only in FIG. I for the sake of simplicity andclarity, it being understood that a similar placement of the elements inall the figures is intended.

Referring now to FIG. 2, igniter 10 is connected in series with resistor12 and switch 14 across power source 16, as in FIG. 1. However, in thisembodiment relay energization winding 18 is connected in parallel withigniter 10 while fuel valve 24 is connected in parallel with resistor 12via relay contacts 22.

In FIG. 3, the same basic circuit is shown as in FIG. I with theexception that the relay energization winding is connected in serieswith the then'nistor l2 and is constructed so as to be current sensitiverather than being voltage sensitive as in the preceding two figures.

FIG. 4 is a modification of the circuit of FIG. 3 with the substitutionof a bimetal actuated switching device 40 for relay 20. Device 40includes a switch 42 which is directly controlled by the movement of abimetallic arm 44 mounted upon a frame 46. Bimetallic arm 44 isillustrated in its cold position wherein switch 42 is maintained closed.The arm 44 is heated by a heating element 48 which is connected inparallel with igniter 10 through one side of a single pole double throwswitch 50. Switch 50 is controlled through a snap-action spring 52 bylever arm 54 which is pivotally mounted upon frame 46 and is biased byspring 56 against the free end of bimetallic arm 44. Thus, in theposition shown, switch 50 provides a closed path from the igniter to theheater element 48 through the lower contacts, as shown. The uppercontacts of switch 50 complete the parallel connection of fuel valve 24to the igniter 10 for energization. The device 40 also includes aholding coil 58 which functions to hold lever arm 54 after it has beenmoved downward by bimetallic arm 44 but is incapable of generating asufficient magnetic field to pull lever 54 down from its unactuatedposition, as illustrated.

In explaining the operation of the circuits described above. a typicalinstallation of the igniter circuit in a central heating furnace systemwill be used. In such a system, switch l4 would function in aconventional manner either as a main power switch, a thermostaticallyoperated ambient temperature sensing device, or a combination powerthermostat switch. It

should also be noted that the placement of switch 14 in the circuit isnot critical to the invention and may be modified in accordance with theparticular operational characteristics desired for various differentinstallations.

Referring to FIG. 1, when the circuit is inactive switches 14 and 22will be open, as shown. Since no power will therefore be applied, thetemperature of both the igniter and the thermistor 12 will be low; and,due to the negative and positive temperature coefficients of resistanceof the igniter and thermistor, respectively, the initial resistance ofthe igniter will be high in comparison with that of the thermistor.

When a demand for heat arises, switch 14 will become closed to therebyfeed power from the source 16 to the series combination of thermistor l2and igniter 10. By visualizing this series circuit as a resistivevoltage divider, the initial voltage drop across the igniter 10 can beseen to be high with respect to the thermistor I2 in direct relation tothe initial value of their respective resistances. At this same time,the igniter begins to increase in temperature which causes the voltagedrop of the igniter to decrease and that of the thermistor to increaseas their resistances vary. Thus, the voltages appearing across the twomajor heat sensing components, namely thennistor l2 and igniter l0, varyin opposite directions in proportion to the temperature of the igniter10.

In the circuit of FIG. I, the voltage drop across the thermistor isapplied to relay energiz'ation winding 18, while the drop across theigniter is fed tothe fuel valve 24 via the relay contacts 22. Therefore,prior to igniter heat-up the small voltage across the thermistor 12 willbe insufficient to energize the relay and the large voltage drop acrossthe igniter will be prevented from actuating the fuel valve by the openswitch contacts 22. In this embodiment, the relay 20 is voltagesensitive and will close contacts 22 only after the resistance of thethennistor, and therefore its voltage drop has been increased by theheating of the igniter to fuel igniting temperatures. As a result,initial fuel flow is prevented until sufficient igniter temperature hasbeen attained, thus assuring immediate ignition whenever fuel if fed tothe burner.

Considering now the operation of the circuit after burner ignition hasbeen established,.it is noted that the igniter 10 is coupled in serieswith the parallel combination of thermistor I2 and energization windingI8, and is coupled in parallel with the series combination of fuel valve24 and relay contacts 22. Therefore, if the igniter devclopes an opencircuit, the flow of current through the thermistor-relay parallelcircuit will cease causing immediate drop out of contacts-22 to therebystop the flow of fuel. In the event of a short circuit in the igniter,the voltage across the fuel valve 24 will drop to zero causing itsimmediate closure. Thus, both open and short circuit protection isprovided to assure failsafe operation at all times.

The circuit of FIG. 2 operates in similar manner as that of FIG. 1. Inthis Figure, however, the relay winding is connected in parallel withthe igniter while the fuel valve is connected across the thermistorthrough the relay switch 22. In operation, the initial large voltagedrop across the igniter upon closure of switch 14 in response to ademand for heat energizes relay 20 to place the fuel valve directlyacross thermistor 12. However, the fuel valve in this embodiment isvoltage sensitive and will not function to establish a flow of fuel tothe burner until the resistance of the thermistor, and therefore, itsvoltage drop has been increased by the heating of the igniter to fueligniting temperatures. Therefore, raw fuel leakage is prevented duringperiods of normal operation.

As in the circuit of FIG. I, igniter short circuit and open cir cuitprotection is provided by the system of FIG. 2. More specifically, theseries connection of the fuel valve and the igniter is designed to blockthe flow of current to the fuel valve if, for any reason, the igniterbecomes an open circuit. This inherently causes the fuel valve to shutoff the fuel flow to the system burner to prevent raw fuel fromcollecting. A short cir cuit in the ignition element will also produceimmediate fuel cutoff since the short circuit will remove theenergization voltage from the relay winding 18 causing the relaycontacts 22 to rent is drawn by the combination. The relay, thermistorand igniter are designed to provide the requisite current flow onlyafter the igniter has reached fuel igniting temperatures to therebyprevent raw fuel leakagelt is noted that in this embodiment the rate ofchange of resistance is greater for the igniter than for the thermistorso that the net rate of change of current flowing therethrough increasesas the temperature of the igniter increases, the thermistor functioningto limit the current through the circuit to prevent thermal runaway bythe igniter.

In the event that the igniter becomes an open circuit, the current flowthrough the relay winding will cease and fuel flow will be blocked bythe fuel valve 24. Similarly, if the igniter becomes a short circuit,the energization voltage across the fuel valve will become zero, therebycausing immediate closure thereof.

Referring now to FIG. 4, when switch I4 is initially closed, currentwill flow through the igniter 10, the thermistor l2 and the holding coil58. The initial voltage drop is primarily across the igniter sincethermistor 12 has an initial low resistance and holding coil 58 isselected to be of a low impedance current sensing type. The largepotential drop across the igniter is fed to heater element 48 throughthe lower set of contacts of switch 50 whereupon the heat generatedthereby causes bimetallic arm 44 to deflect away from the heater 48.This causes switch 42 to open and then, by movement of lever arm 54,causes switch 50 to snap to its upper position whereupon the heaterelement 48 is removed from the circuit. During this series of events theigniter 10 becomes hot thereby changing its resistance to permit alarger flow of current through the holding coil 58.. The coil thenproduces sufficient magnetization to maintain lever arm 54 in itsdeflected position, thereby maintaining the upper contacts of switch 50closed, while bimetallic arm 44 begins to return to its cold or restposition. The bimetal, as it continues to return to rest, then closesswitch 42 to complete the parallel connection of the fuel valve with theigniter through switch 42 and the upper contacts of switch 50. Thesystem is designed so that the aforedescribed series of events willoccur in no shorter time than the time required for igniter 10 to comeup to fuel ignition temperatures. This safety factor is furtherincreased by the fact that only after the resistance of igniter 10 hasbeen decreased by the generation thereby of fuel ignition temperatureswill sufficient current flow through the holding coil 58 to maintainlever arm 54 and, therefore, switch 50 in their actuated positions. 1

After ignition, if the igniter in the'circuit of FIG. 4 develops a shortcircuit, the voltage across the fuel valve will become zero therebycutting ofi the flow of fuel to the burner. Similarly, the voltage tothe fuel valve will be removed by the switch 50 if the igniter developsan open circuit since the current flow through the holding coil willcease and lever arm 54 will be released.

Therefore, there is provided in all of the illustrated embodiments anelectric igniter system which is permitted to cycle continuously inresponse to a demand for heat while providing raw fuel leakageprevention as well as igniter short or open circuit fail-safe operation.

In as much as the present invention is subject to many variations,modifications and changes in detail, it is intended that all mattercontained in the foregoing description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

. What is claimed is:

I. An electric ignition system for a fuel burner comprising input meansadapted to be connected with an electrical power source,

an electric ignitor coupled to said input means and adapted to belocated within igniting proximity of the burner,

said igniter having a negative temperature coefficient of resistance,

a resistor having a positive temperature coefficient of resistancecoupled in series with said igniter and adapted to be heated thereby,

an electrically operated valve adapted to control a flow of fuel to theburner, and

means coupled to said ignitor, said resistor, and said valve andresponsive to the relative resistances of said ignitor and said resistorfor enabling energization of said valve to establish a flow of fuel whensaid ignitor is above fuel igniting temperatures and for disabling saidvalve upon failure of said ignitor.

2. The invention as recited in claim 1 wherein said means includes anelectric relay having an energization winding and a switch responsivethereto, said switchbeing coupled in series with said valve to form aseries circuit.

3. The invention as recited in claim 2 wherein said series circuit iscoupled in parallel with said resistor, and said energization winding iscoupled in parallel with said ignitor.

4. The invention as recited in claim 2 wherein said series circuit iscoupled in parallel with said igniter and said energization winding iscoupled in parallel with said resistor.

5. The invention as recited in claim 2 wherein said series circuit iscoupled in parallel with said igniter and said energization winding iscoupled in series with said resistor and said ignitor.

6. The invention as recited in claim 5 further including thermallyresponsive switch means connected with said valve and said relay, saidthermally responsive switch means including a bimetallic element and anelectric heater located within heating proximity of said bimetallicelement and coupled with said ignitor.

7. The invention as recited in claim 6 wherein said thermally responsiveswitch means further includes a normally closed switch coupled in serieswith said valve and said relay switch and adapted to be opened by saidbimetallic element in response to energization of said electric heater,and wherein said relay further includes a normally closed switchconnected in series with said electric heater, said relay beinginitially actuated by said bimetallic element and being maintained inits actuated position by said energization winding whenever said ignitoris above fuel igniting temperature.

8. In an electric ignition system for a fuel burner, the combinationcomprising input means adapted to be connected to an electrical powersource,

an electric ignitor adapted to be located adjacent the burner andcoupled to said input means,

said ignitor having a negative temperature coefficient of resistancethereby producing a temperature dependent voltage drop thereacross, anelectrically operated valve adapted to control a flow of fuel to theburner and being electrically coupled with said electric ignitor andsaid input means for energization by said power source when said ignitoris above fuel ignition temperature, switch means connecting saidelectrically operated valve with said ignitor whereby said valve isdeenergized to thereby inhibit fuel flow therethrough whenever saidignitor develops an electrical short circuit, and circuit meansconnected with said ignitor and said switch means for disabling saidvalve to thereby inhibit fuel flow therethrough when said ignitordevelops an electrical open circuit. 9. The invention as recited inclaim 8 wherein saidswitch means connects said electrically operatedvalve across said igniter and wherein said circuit means is connected inseries circuit with said ignitor said switch means and said circuitmeans cooperating to inhibit actuation of said valve until said ignitorreaches fuel ignition temperatures.

10. The invention as recited in claim 9 wherein said circuit meanscomprises a resistor having a positive temperature coefficient ofresistance and an energization winding of an electric relay coupled toform an electrical parallel circuit, and wherein said switch meanscomprises a normally open switch adapted to be closed in response toenergization of said winding, said switch being coupled in series withsaid electrically operated valve.

11. The invention as recited in claim 9 wherein said circuit meanscomprises a resistor having a positive temperature coefficient ofresistance and an energization winding of an electric relay coupled toform a series circuit, and wherein said switch means comprises anormally open switch adapted to be closed upon energization of saidwinding, said switch being coupled in series with said electricallyoperated valve.

12 The invention as recited in claim 11 wherein said energizationwinding is a holding coil, and wherein said switch means furtherincludes thermally responsive switching means connected with saidelectrically operated valve and said normally open switch, saidthermally responsive switch means including a bimetallic element and anelectric heater located within heating proximity of said bimetallicelement and coupled with said ignitor.

13. The invention as recited in claim 12 wherein said thermallyresponsive switching means further includes a normally closed switchcoupled in series with said electrically operated valve and saidnormally open switch and adapted to be opened by said bimetallic elementin response to energization of said electric heater, and wherein saidrelay further includes a normally closed switch connected in series withsaid electric heater, said relay being initially actuated by saidbimetallic element and being maintained in its actuated position by saidholding coil whenever said ignitor is above fuel igniting temperatures.

1. An electric ignition system for a fuel burner comprising input meansadapted to be connected with an electrical power source, an electricignitor coupled to said input means and adapted to be located withinigniting proximity of the burner, said igniter having a negativetemperature coefficient of resistance, a resistor having a positivetemperature coefficient of resistance coupled in series with saidigniter and adapted to be heated thereby, an electrically operated valveadapted to control a flow of fuel to the burner, and means coupled tosaid ignitor, said resistor, and said valve and responsive to therelative resistances of said ignitor and said resistor for enablingenergization of said valve to establish a flow of fuel when said ignitoris above fuel igniting temperatures and for disabling said valve uponfailure of said ignitor.
 2. The invention as recited in claim 1 whereinsaid means includes an electric relay having an energization winding anda switch responsive thereto, said switch being coupled in series withsaid valve to form a series circuit.
 3. The invention as recited inclaim 2 wherein said series circuit is coupled in parallel with saidresistor, and said energization winding is coupled in parallel with saidignitor.
 4. The invention as recited in claim 2 wherein said seriescircuit is coupled in parallel with said igniter and said energizationwinding is coupled in parallel with said resistor.
 5. The invention asrecited in claim 2 wherein said series circuit is coupled in parallelwith said igniter and said energization winding is coupled in serieswith said resistor and said ignitor.
 6. The invention as recited inclaim 5 further including thermally responsive switch means connectedwith said valve and said relay, said thermally responsive switch meansincluding a bimetallic element and an electric heater located withinheating proximity of said bimetallic element and coupled with saidignitor.
 7. The invention as recited in claim 6 wherein said thermallyresponsive switch means further includes a normally closed switchcoupled in series with said valve and said relay switch and adapted tobe opened by said bimetallic element in response to energization of saidelectric heater, and wherein said relay further includes a normallyclosed switch connected in series with said electric heater, said relaybeing initially actuated by said bimetallic element and being maintainedin its actuated position by said energization winding whenever saidignitor is above fuel igniting temperature.
 8. In an electric ignitionsystem for a fuel burner, the combination comprising input means adaptedto be connected to an electrical power source, an electric ignitoradapted to be located adjacent the burner and coupled to said inputmeans, said ignitor having a negative temperature coefficient ofresistance thereby producing a temperature dependent voltage dropthereacross, an electrically operated valve adapted to control a flow offuel to the burner and being electrically coupled with said electricignitor and said input means for energization by said power source whensaid ignitor is above fuel ignition temperature, switch means connectingsaid electrically operated valve with said ignitor whereby said valve isdeenergized to thereby inhibit fuel flow therethrough whenever saidignitor develops an electrical short circuit, and circuit meansconnected with said ignitor and said switch means for disabling saidvalve to thereby inhibit fuel flow therethrough when said ignitordevelops an electrical open circuit.
 9. The invention as recited inclaim 8 wherein said switch means connects said electrically operatedvalve across said igniter and wherein said circuit means is connected inseries circuit with said ignitor, said switch means and said circuitmeans cooperating to inhibit actuation of said valve until said ignitorreaches fuel ignition temperatures.
 10. The invention as recited inclaim 9 wherein said circuit means comprises a resistor having apositive temperature coefficient of resistance and an energizationwinding of an electric relay coupled to form an electrical parallelcircuit, and wherein said switch means comprises a normally open switchadapted to be closed in response to energization of said winding, saidswitch being coupled in series with said electrically operated valve.11. The invention as recited in claim 9 wherein said circuit meanscomprises a resistor having a positive temperature coefficient ofresistance and an energization winding of an electric relay coupled toform a series circuit, and wherein said switch means comprises anormally open switch adapted to be closed upon energization of saidwinding, said switch being coupled in series with said electricallyoperated valve. 12 The invention as recited in claim 11 wherein saidenergization winding is a holding coil, and wherein said switch meansfurther includes thermally responsive switching means connected withsaid electrically operated valve and said normally open switch, saidthermally responsive switch means including a bimetallic element and anelectric heater located within heating proximity of said bimetallicelement and coupled with said ignitor.
 13. The invention as recited inclaim 12 wherein said thermally responsive switching means furtherincludes a normally closed switch coupled in series with saidelectrically operated valve and said normally open switch and adapted tobe opened by said bimetallic element in response to energization of saidelectric heater, and wherein said relay further includes a normallyclosed switch connected in series with said electric heater, said relaybeing initially actuated by said bimetallic element and being maintainedin its actuated position by said holding coil whenever said ignitor isabove fuel igniting temperatures.